1. Field of the Invention
The present invention relates to an optical waveguide structure of an intersecting portion in a circuit structure having a portion where optical waveguides substantially intersect with each other.
2. Description of the Related Art
In accordance with development of researches on silicon photonics in which the mature process technique in a silicon LSI can be used, recently, it is possible to realize an optical waveguide which is very fine, and which produces a low loss even in a sharp bend. Therefore, miniaturization and low power consumption of a transmitting/receiving module and system for optical communication, and introduction and integration of optical interconnections into a silicon LSI are becoming enabled. A potential candidate of such an optical waveguide is a silicon (Si) thin line waveguide which can be formed on a silicon-on-insulator (SOI) substrate by a relatively simple technique.
In order to suppress the cost while satisfying increasing demands for a signal amount and a communication speed, it is important to improve the integration degree of an optical waveguide circuit. Therefore, waveguides are requested not only to be arranged in parallel, but also to intersect with each other. In an optical waveguide typified by an Si thin line waveguide, however, it is known that signal lights respectively propagating through two or more waveguides which intersect with each other are scattered and reflected, and interfere with each other in intersecting portions, thereby causing a high loss or crosstalk. It is difficult to design an optical waveguide circuit in which many intersecting portions must be formed.
In a conventional intersecting optical waveguide circuit, for example, two Si thin line waveguides intersect with each other, and signal lights propagate beyond the intersecting portion. At this time, scattering, reflection, interference, and crosstalk of lights occur in the intersecting portion, so that a loss of about 1.5 dB and crosstalk of −9.2 dB may occur in the propagation properties of one of the signals lights. Since the portion has a point-symmetric structure, such a high loss and crosstalk are naturally produced in the propagating signal lights in both the intersecting waveguides.
On the other hand, also a technique is known in which a waveguide in, for example, an intersecting portion is formed into an oval shape, thereby reducing the levels of a loss and crosstalk (for example, see Non-patent Reference 1). In this structure, light scattering in the intersecting portion is suppressed, and excellent propagation in which a loss of about 0.1 dB and crosstalk of about −25 dB or less occur in a signal light propagating in the waveguide having an oval shape is enabled.
In this structure, however, the propagation property of the optical waveguide which is not formed into an oval shape is impaired as compared with the conventional structure, with the result that the loss and crosstalk are high. Moreover, the Non-patent Reference describes that, when it is attempted that the oval structure is applied to both of two intersecting waveguides to reduce the losses of signal lights in the waveguides, the loss is 1.2 dB or at the level in the vicinity of 1.5 dB which is attained in the case where no countermeasure is taken.
In the structure, namely, the propagation property of only one of the intersecting optical waveguides can be improved, but it is considered that the propagation properties of both of the two intersecting optical waveguides are hardly improved. High-density integration of optical interconnections which require many intersecting portions is hardly realized by intersecting optical waveguides having such a property.
In a portion where optical waveguides intersect with each other, it is difficult that, not only a signal light propagating in one of the waveguides, but also all signal lights propagating in the waveguides are enabled to propagate at a low loss and low crosstalk. When all signal lights propagating in the waveguides cannot propagate at a low loss and low crosstalk, intersecting portions are hardly included in the circuit design, so that the degree of freedom in the circuit design is lowered and high-density integration of optical interconnections is impeded.